Viral vectors that are able to induce both strong and long-lasting immune responses may be required as effective vaccines for human immunodeficiency virus type-I (HIV-1) infection. The recent findings that viral vectors expressing HIV-1 and/or simian immunodeficiency virus (SIV) proteins are able to protect macaques from a AIDS-like disease are promising, but their failure to protect from SIV infection also indicate that further improvements are required. We have recently developed new potential HIV-1 vaccines based on highly attenuated, replication-competent rabies virus (RV), influenza A virus (FluA), and replication-deficient Newcastle disease virus (NDV). RV and FluA vectors were able to induce both strong humoral and cellular immune response against the expressed HIV-1 antigens in mice and they are therefore excellent candidates for potential HIV-1 vaccines. In a combined effort of two research groups to further develop an efficacious vaccine against AIDS, three different viral vectors expressing HIV-1 or SIV Gag or envelope protein will be utilized as vaccination vehicles in a new prime/boost approach. Priming with naked DNA followed by a boost with a replication-deficient viral vectors has been shown previously to enhance the cellular immune response against HIV-1 Gag. So far, very little information is available for prime/boost immunization against HIV-1 utilizing different viral vectors. To analyze this new approach, mice will be primed with one viral vector followed by one or two boosts with another vaccine vector expressing the same antigen (Gag or Env). Several immunizations with the same vector will serve as controls. At each step during immunization both the cellular and humoral HIV-1 specific immune responses will be determined by cytotoxic T-lymphocytes (CTLs), ELISPOTs, tetrameric staining, IgG and IgA ELISA, and HIV-1 neutralization assays. In addition, we will determine if efficient boosting can also be achieved with a prime/boost approach using two RV-based vectors expressing SIV or HIV-1 antigens that contain two serologically unrelated rhabdoviral glycoproteins. The data from this research project will indicate if: I) immunization with different viral vectors enhance the SIV/HIV-1 specific humoral and/or cellular responses compared to multiple applications of the same vector, ii) immunological differences occur between replication-competent and non-competent viral vectors used for priming or boosting, iii) the possibility of combining the advantages of two different viral vectors such as the induction of a vigorous cellular response by one vaccine and mucosal immunity by the other.